Seed layer processes for MOCVD of ferroelectric thin films on high-k gate oxides

ABSTRACT

A method of forming a ferroelectric thin film on a high-k layer includes preparing a silicon substrate; forming a high-k layer on the substrate; depositing a seed layer of ferroelectric material at a relatively high temperature on the high-k layer; depositing a top layer of ferroelectric material on the seed layer at a relatively low temperature; and annealing the substrate, the high-k layer and the ferroelectric layers to form a ferroelectric thin film.

RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.10/015,817, filed on Oct. 30, 2001, for High k Gate Oxides with BufferLayers of Ti for MFOS One Transistor Memory Applications; ThisApplication is a division of patent application Ser. No. 10/453,831 forMethod of Forming Ferroelectric Thin Film on a High-K Layer, now U.S.Pat. No. ______, granted ______, which is a Division of U.S. patentapplication Ser. No. 10/020,868, filed Dec. 12, 2001, for Seed Layer forMOCVD Ferroelectric Thin Films on High-K Gate Oxides, now U.S. Pat. No.6,897,074, granted May 24, 2005.

FIELD OF THE INVENTION

This invention relates to ferroelectric thin film processes,ferroelectric memory device structures and integrated processes forferroelectric non-volatile memory devices, and specifically to a methodof fabricating a ferroelectric thin film formed on a high-k gate oxide.

BACKGROUND OF THE INVENTION

Metal, ferroelectrics, oxide, and silicon (MFOS) transistorferroelectric memory devices have been proposed in the relatedapplications. In order to provide a MFOS transistor memory device havingdesirable characteristics, the oxide must not react with nor diffuseinto the ferroelectric and silicon substrate. On the other hand,ferroelectric thin films deposited on an oxide layer should have goodferroelectric properties for use in a memory transistor. Homogeneousferroelectric thin films having desirable ferroelectric properties aredifficult to deposit on gate oxides because of the interface mismatchbetween the high-k gate oxide and the ferroelectric materials. Themismatch results in random ferroelectric, such as PGO, thin films andcoarser surface roughness. Seed layer processes have been developed forMFOS transistor ferroelectric memory applications to resolve theinterface mismatch.

SUMMARY OF THE INVENTION

A method of forming a ferroelectric thin film on a high-k layer includespreparing a silicon substrate; forming a high-k layer on the substrate;depositing a seed layer of ferroelectric material at a relatively hightemperature on the high-k layer; depositing a top layer of ferroelectricmaterial on the seed layer at a relatively low temperature; andannealing the substrate, the high-k layer and the ferroelectric layersto form a ferroelectric thin film.

It is an object of the invention to provide a seed layer of FE materialto enhance the FE deposition process.

A further object of the invention is to provide a FE device which is notdegraded by an interface mismatch.

Another object of the invention is to provide a FE device using a seedlayer processes to deposit homogeneous ferroelectric thin films on ahigh-k gate oxide, such as ZrO₂, HfO₂ and (Zr_(x)Hf_(1-x))O₂, in a MFOStransistor ferroelectric memory application.

Another object of the invention is to provide a MFOS one transistordevice.

This summary and objectives of the invention are provided to enablequick comprehension of the nature of the invention. A more thoroughunderstanding of the invention may be obtained by reference to thefollowing detailed description of the preferred embodiment of theinvention in connection with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an as-deposited PGO thin film without a seed layer.

FIG. 2 depicts the PGO thin film of FIG. 1 after annealing.

FIG. 3 depicts X-ray patterns of a PGO seed layer, and of PGO thin filmas-grown and after annealing.

FIG. 4 depicts an as-grown PGO thin film deposited on a high-k layerwith a seed layer.

FIG. 5 depicts the PGO thin film of FIG. 4 after annealing.

FIG. 6 depicts a C-V curve of a PGO MFOS capacitor constructed accordingto the method of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to explain the fabricate a ferroelectric layer on a high-klayer, a ferroelectric (FE) thin film metal-ferroelectric-oxide-silicon(MFOS) structure has been selected for incorporation into a onetransistor memory device as an example of the method of the invention.To obtain adequate electrical properties in a MFOS transistor memorydevice, a seed layer is fabricated according to the method of theinvention for a MFOS transistor ferroelectric memory application. Asmooth ferroelectric lead germanium oxide (Pb₅Ge₃O₁₁) (PGO) thin filmsmay be deposited on a high-k gate oxide, such as ZrO₂, HfO₂, or (Zr_(x),Hf_(1-x))O₂, which PGO thin film has a low surface roughness and uniformthickness using the seed layer MOCVD method of the invention.

THE METHOD OF THE INVENTION

In the preferred embodiment of the method of the invention, a siliconwafer of the P type is used as a substrate of a MFOS one transistormemory cell. The silicon wafer is cleaned using SC1+SC2; where SC1 is amixture of 5500 ml of deionized water, 1100 ml of NH₄OH and 1100 ml ofH₂O₂, and where SC2 is a mixture of 6000 ml of deionized water, 1000 mlof HCl and 1000 ml of H₂O₂. The surface oxide is removed by an HF dipetch. A layer of high-k material, such as HfO₂ and (Zr_(0.5),Hf_(0.5))O₂ thin film, having a thickness of between about 3.5 nm to 15nm are deposited on the silicon substrate, by sputtering in thepreferred embodiment. The silicon wafer, with the HfO₂ and (Zr_(0.5),Hf_(0.5))O₂ layers, are annealed at between about 500° C. to 550° C. ina pure oxygen atmosphere to achieve full oxidation. An oxide MOCVDreactor is used for the growth of a layer having a thickness of betweenabout 200 nm to 300 nm of c-axis oriented Pb₅Ge₃O₁₁ (PGO) thin film onthe (Zr_(0.5), Hf_(0.5))O₂ layer. A top electrode of platinum, having athickness of about 100 nm, is deposited by an electrical beamevaporation technique.

For the MOCVD deposition of FE material, [Pb(thd)₂] and [Ge(ETO)₄],where thd is C₁₁H₁₉O₂ and ETO is OC₂H₅, in a molar ratio of about 5.0:3to 5.5:3, are dissolved in a mixed solvent of butyl ether ortetrahydrofuran, isopropanol and tetraglyme in the molar ratio of about8:2:1 to form a precursor solution. The precursor solution has aconcentration of about 0.1 M/L of PGO. The solution is injected into avaporizer at temperature in the range of between about 180° C. to 240°C., by a pump at a flow rate of between about 0.05 ml/min to 0.20ml/min, to form the precursor gases. The feed line is maintained at atemperature of between about 185° C. to 245° C.

A FE seed layer MOCVD and annealing process, in the preferredembodiment, includes deposition of a layer of c-axis oriented PGO thinfilm, deposited on the high-k (Zr_(0.5), Hf_(0.5))O₂, as follows: thedeposition temperatures is between about 500° C. to 540° C. and thepressure is between about 1 torr to 5 torr; the oxygen partial pressuresis between about 20% to 30%; the vaporizer temperature is between about180° C. to 200° C.; the solution delivery rates is between about 0.05ml/min to 0.1 ml/min; and the deposition time is between about 5 minutesto 20 minutes.

The main body, or top FE layer, of PGO is deposited on the seed layer ata relatively lower temperature as follows: the deposition temperaturesis between about 380° C. and 420° C. and the pressure is between about 5torr to 10 torr; the oxygen partial pressure is between about 30% to40%; the vaporizer temperature is between about 200° C. to 240° C.; thesolution delivery rate is between about 0.1 ml/min to 0.2 ml/min; andthe deposition time is between about one hour to three hours, dependingon the desired film thickness.

The post-deposition annealing temperature is between about 520° C. to560° C. for between about thirty minutes and one hour in a pure oxygenatmosphere. The phases of the films are identified using x-raydiffraction. The capacitance of the PGO MFOS capacitors is measuredusing a Keithley 182 CV analyzer.

Results

In order to form a smooth PGO thin film having low surface roughness andcontaining relatively smaller particles, a PGO thin film must bedeposited at a relatively lower deposition temperature, and thenannealed at a relatively higher temperature to promote uniform graingrowth. Experimental results demonstrate that smooth, amorphous PGO thinfilms became very rough following a high temperature anneal, as shown bythe SEM photos of FIGS. 1 and 2. The reason for the rough surface isthat there are not enough crystal nuclei in the PGO thin film depositedat the lower temperature to promote uniform grain growth, which resultsin a high surface roughness.

The seed layer MOCVD process of the method of the invention wasdeveloped to solve this problem. A PGO seed layer is deposited at arelatively high temperature and at a relatively low deposition pressure.The consequential low deposition rate and relatively low oxygen partialpressure avoid any gas phase reaction, which may result in the formationof undesirable particles. Formation of a homogeneous and continuousc-axis oriented PGO seed layer is required for the subsequent thin filmgrowing steps. FIG. 3 depicts the x-ray pattern of a PGO seed layer,generally at 10, demonstrating the presence of a single-phase c-axis PGOseed layer.

The next step in the method of the invention is to grow an amorphous PGOthin film on the seed layer at a relatively low deposition temperature,and to then anneal the PGO thin film at a relatively high temperature tomake the PGO thin film fully crystallize. Because the PGO seed layerprovides homogeneous crystal nuclei for PGO grain growth during theannealing process, a smooth and fully crystallized PGO thin film isformed. FIG. 3 also depicts the x-ray patterns of PGO thin filmsas-grown 12 and after annealing 14.

FIG. 4 is a SEM photo of an as-grown PGO thin film on (Zr_(x),Hf_(1-x))O₂, while FIG. 5 is a SEM photo of the PGO thin film afterannealing. As-grown PGO thin films are very smooth and have very lowsurface roughness. After annealing at about 540° C., the grain size ofthe PGO thin film formed according to the method of the inventionincreases, but the PGO thin film is still smooth, and the annealing hasalmost no affect on the surface roughness of the thin film.

Platinum top electrodes are deposited on the PGO thin films made by theseed layer MOCVD process of the method of the invention to form a MFOScapacitor. FIG. 6 depicts the C-V curve of a PGO MFOS capacitor formedaccording to the seed layer MOCVD process of the method of theinvention. The memory window of 1.5 V to 2.0 V is easily measured. Ahomogeneous ferroelectric thin films may be deposited on a high-k gateoxide while maintaining excellent ferroelectric properties according tothe seed layer method of the invention. The seed layer may be depositedby MOCVD, sputtering, MOD, and sol-gel etc. methods. A homogeneous andsmooth PGO thin film, having low surface roughness and uniform thicknessmay be deposited on a (Zr_(x), Hf_(1-x))O₂ layer using the seed layerMOCVD process method of the invention. The memory windows of PGO MFOScapacitors are measured in a range of about 1.5 V-2.0 V.

Thus, a seed layer processes for MOCVD ferroelectric thin filmsdeposited on high-k gate oxides has been disclosed. It will beappreciated that further variations and modifications thereof may bemade within the scope of the invention as defined in the appendedclaims.

1-8. (canceled)
 9. A method of forming a ferroelectric thin film on ahigh-k layer, comprising: preparing a silicon substrate; forming ahigh-k layer on the substrate and annealing the high-k layer to form anoxide layer; preparing a PGO precursor solution; depositing a seed layerof ferroelectric material on the high-k oxide layer, wherein saiddepositing a seed layer includes depositing a layer of c-axis orientedPGO thin film at a temperatures of between about 500° C. to 540° C., ata pressure of between about 1 torr to 5 torr, in an atmosphere having anoxygen partial pressures of between about 20% to 30%, at a vaporizertemperature of between about 180° C. to 200° C.; wherein the precursorsolution has a delivery rate of between about 0.05 ml/min to 0.1 ml/min,for a deposition time of between about 5 minutes to 20 minutes;depositing a top layer of ferroelectric material on the seed layer at arelatively low temperature, wherein said depositing a top ferroelectriclayer includes depositing a PGO thin film at a deposition temperaturesof between about 380° C. and 420° C., wherein the deposition pressure isbetween about 5 torr to 10 torr, in an atmosphere having an oxygenpartial pressure of between about 30% to 40%, at a vaporizer temperatureof between about 200° C. to 240° C., wherein the precursor solutiondelivery rate is between about 0.1 ml/min to 0.2 ml/min; and wherein thedeposition time is between about one hour to three hours; and annealingthe substrate, the high-k layer and the ferroelectric layers to form aferroelectric thin film, wherein said annealing includes annealing at atemperature of between about 520° C. to 560° C. for between about thirtyminutes and one hour in a pure oxygen atmosphere.
 10. The method ofclaim 9 wherein said preparing a ferroelectric precursor solution fordeposition by MOCVD, includes preparing a solution of [Pb(thd)₂] and[Ge(ETO)₄], where thd is C₁₁H₁₉O₂ and ETO is OC₂H₅, in a molar ratio ofabout 5.0:3 to 5.5:3, are dissolved in a mixed solvent, consisting ofsolvents taken from the group of solvents consisting of butyl ether, andtetrahydrofuran, isopropanol and tetraglyme, in the molar ratio of about8:2:1, wherein the solution has a concentration of about 0.1 M/L of PGO,and wherein the solution is injected into a vaporizer at temperature inthe range of between about 180° C. to 240° C., by a pump at a flow rateof between about 0.05 ml/min to 0.20 ml/min, to form the precursor gas,and wherein a MOCVD feed line is maintained at a temperature of betweenabout 185° C. to 245° C.
 11. The method of claim 9 wherein said forminga high-k layer includes forming a layer of material taken from the groupof material consisting of HfO₂ and (Zr_(0.5), Hf_(0.5))O₂, to athickness of between about 3.5 nm to 15 nm.
 12. The method of claim 9wherein said preparing the silicon substrate includes cleaning thesubstrate using SC1+SC2; where SC1 is a mixture of 5500 ml of deionizedwater, 1100 ml of NH₄OH and 1 100 ml of H₂O₂, and where SC2 is a mixtureof 6000 ml of deionized water, 1000 ml of HCl and 1000 ml of H₂O₂; andwhich further includes removing any surface oxide by an HF dip etch.13-21. (canceled)